I think it's only for paid members so far. It lets you add little notes to anyone and then puts a little * next to the usernames that you can hover over to see your notes.

D!

Also, lookie here (http://community.livejournal.com/brb_gallifrey/127793.html?thread=7231281#t7231281) for Frobisher and Six, in case you haven't seen.